This invention relates to a sensible and latent heat exchange wheel by which heat and moisture are exchanged between a supply air stream and an exhaust air stream, and more particularly, the invention relates to a higher strength desiccant paper suitable for fabricating into an enthalpy wheel for aiding in regulating the temperature and humidity of ambient air.
Enthalpy wheels or heat exchange wheels are mounted across a supply air stream and an adjacent exhaust air stream. The wheels rotate and continuously remove heat and/or moisture from one stream and return heat and/or moisture to the adjacent stream. The wheels are usually fabricated to provide a multiplicity of parallel pores or openings such as a honeycomb structure through which the air passes. The wheels can be formed from coated material such as aluminum, plastic and paperboard or desiccant paper having one side corrugated and one side flat. The wheels are formed by winding the coated material or desiccant paper into the wheel shape to provide air passageways parallel to the axis of the wheel.
However, one of the problems with the desiccant paper is that it has to be relatively thick in order to have the strength necessary for the wheel. This results in resistance to flow of air through the wheel requiring higher air pressures and consequently more power to operate. Further, thicker paper requires that wheels have to be larger and thus heavier, often compromising the design of the wheel. Thus, it will be seen that it is desirable to have thin, strong desiccant paper to minimize pressure drop and to obtain maximum area of desiccant paper per unit of volume to improve efficiency.
The desiccant paper is comprised of a matrix or media capable of absorbing moisture and/or thermal energy from one stream and upon further rotation of the wheel, releasing the moisture and/or thermal energy to the adjacent stream which is to be conditioned. For example, the wheel can be used to recover moisture and heat from exhaust air at relatively high temperature for transfer to incoming air which is relatively dry and cool. Also, the wheel can be used to dehumidify and cool a moist, hot incoming stream by extracting moisture and heat from such stream and transferring the same to an exhaust stream from an air conditioned building that is relatively cooler and drier.
To improve indoor air quality, there is a great need to increase levels of outdoor air ventilated into buildings. However, it is important to minimize the cost associated with the introduction of outside air. For example, in winter, introducing cold, dry outdoor air to a building increases the heating load and adds to the fuel requirement. In summer, introduction of warm, humid outdoor air adds greatly to the air conditioning costs. Thus, to minimize costs associated with introduction of fresh air, heat exchangers such as rotating wheels have been employed to recover or reject, as needed, a portion of the sensible heat from the indoor air stream through exchange with the outdoor air stream. Further, some of the heat exchangers provide for latent heat exchange by incorporating a desiccant into the heat exchange surface. Thus, in summer, moisture in an outdoor air stream being introduced to a building is removed and transferred to drier indoor air stream exiting the building. Conversely, in winter, moisture from air being ventilated from the building is adsorbed by the desiccant and subsequently desorbed into the drier outside air being introduced to the building. Thus, such heat exchangers transfer both sensible and latent heat and are often referred to as a total energy exchanger or an enthalpy exchanger. However, while these devices are very desirable for purposes of conserving energy and reducing heating and cooling bills, they have not found widespread acceptance.
Prior references recognize the need for safe heat exchange devices. U.S. Pat. No. 4,093,435 discloses a rotary regenerative total heat energy exchanger having an asbestos-free heat exchange media formed by spirally winding flat and corrugated webs of aluminum foil and/or fully bleached, 45 lb Kraft paper constituted by 84% fibers and 16% salts. The patent notes that single-faced corrugated paperboard is made using a conventional silicate-type glue between a flat ply and a corrugated ply and that each ply is a Kraft paper treated with a mixture of ammonium sulfide and diammonium phosphate. However, such constituents can result in biological growth because the paper is primarily wood pulp based. Further, the wood pulp based papers tend not to age well in continuous long term use.
U.S. Pat. Nos. 3,844,737; 4,012,206 and 4,134,743 disclose the use of zeolites as a desiccant in a regeneratable heat exchange apparatus. The apparatus comprises a rotatable wheel fabricated from a paper support material comprised of cellulose fibers, glass fibers, asbestos, synthetic plastic fibers and zeolite desiccant in the amount of 1 to 90 wt. %.
Adsorbents and fibrous materials and desiccant wheels are disclosed in U.S. Pat. Nos. 3,024,867; 4,012,206; 4,769,053; 3,266,973; 3,338,034 and 4,255,171.
In spite of these disclosures, there is still a need for an improved desiccant paper and wheel formed therefrom having improved mechanical strength yet sufficiently thin for reduced pressure drop to provide a high level of sensible as well as latent heat exchange in a compact, efficient and low cost unit.